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Iljitsch van Beijnum wrote: > On 18 mei 2009, at 23:17, Joe Touch wrote: > >>> Not necessarily. PMTUD for tunnels is just an optimization. There is no >>> reason why you couldn't take full size inner packets and encapsulate >>> them in an outer packet that is then fragmented. > >> You wouldn't know to do that unless you had a different idea of the MTU >> at the endpoint. Or are you suggesting that ROHC do this (I don't think >> it would)... > > Suppose the MTU is 1500, IPsec takes 50 bytes and ROHC may add 4 bytes > worst case but saves 32 bytes on average (completely made up numbers). Now: > > - packets smaller than 1500 - 50 - 4 = 1446 bytes can always be carried > without trouble, so nothing special happens > > - packets larger than 1500 - 50 + 32 = 1482 bytes can never be carried > without fragmentation, so send a too big message. > > - packets between 1446 and 1482 bytes will usually compress to something > that can be carried without fragmentation, so we don't send a too big > message. However, they may not compress, in which case the resulting > packet is larger than 1500 bytes. We then fragment the packet upon > sending and reassemble before detunneling. > > The whole thing can be made dynamic so in the range 1446 - 1482 we don't > always fragment but rather when we see that we fragment too much, we > lower the MTU value reported back to sources until the level of > fragmentation is within reason. That's fine if that's what ROHC already does. It's not fine if this is what you expect and ROHC dumps the too-big packets between 1446 and 1482 on the floor. I agree that some fragmentation like this might actually be a big win. However, we're also assuming that the receiver reassembly happens fast enough that the packets aren't delayed - i.e., if they are sent 'slow path' in the receiver (if there is such a thing, and there could be, e.g., on hardware-assisted hosts), then this would muck up the ROHC mechanism... Joe -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.9 (MingW32) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org iEYEARECAAYFAkoR1SYACgkQE5f5cImnZrutOACg4u3dAoVUCBdYqzZyZPy+UDMn 5PQAoNyAcTYdF6vfg7Xo6mtYRfh+JuQj =CAik -----END PGP SIGNATURE-----
